Bilevel printing devices are characterized by having only two gray levels, i.e., black and white, with fixed sized spots arranged in a prearranged geometric pattern. For bilevel printing devices, the manner in which image data is processed generally depends on the type of data that is predominant e.g., text, continuous tone or halftone.
For the reproduction of continuous tone images, a number of processes have been developed based on pseudo halftone concepts, which mimic the traditional halftone printing process, wherein a variety of image gray levels are achieved through dot size variations. In these pseudo halftone processes, each halftone spot is made up of a matrix of binary picture elements, commonly referred to as pixels or pels, and the effective resolution of the reproduction system is correspondingly reduced.
There are other processes which do not mimic the halftone process, but instead utilize localized information in deciding whether a particular spot should be printed or not. Error diffusion is one such technique having good detail capabilities; however, it can also generate unwanted artifacts. Most digital imaging systems using digital halftoning have concentrated on halftoning algorithms that keep these artifacts or grain patterns to a minimum. This has resulted in very complex algorithms.
The present technique uses a simple error diffusion algorithm in conjunction with a post processor to reduce the undesirable noise or artifacts. This combination of two simple techniques allows the process to be less complex than using one more sophisticated halftoning algorithm.